The administration by inhalation of medicaments in dry powder form is well known. Devices for the metering and dispensing of measured doses of medicament from a reservoir have also been described previously (for example in UK Patent No 2041763). Such devices typically comprise a medicament reservoir and a metering chamber with a volume chosen such that, when filled, the chamber contains the desired weight of medicament for one dose. Filling of the metering chamber is generally accomplished under the influence of gravity, the chamber typically being located at the bottom of the reservoir. Such devices have the disadvantage that variations in the density of the metered powder can easily occur resulting in inaccurate or inconsistent dosing. The packing density of the powder may also depend on the weight of powder remaining in the reservoir, leading to a gradual reduction in the dose delivered by the device. In addition, the dose metered is strongly dependent on the orientation of the device.
Furthermore, when the medicament to be administered is hygroscopic, great care must be taken to ensure that water does not contaminate the medicament because this may cause lumps of medicament to form which can clog the device and lead to inconsistent dosing. The problem of water uptake by hygroscopic medicaments is overcome to some extent by supplying them in gelatin capsules which are punctured immediately prior to administration in a suitable device (for example the device of UK Patent 1122284). However, a fresh capsule must be inserted into the device for each dose, and the volume occupied by a month's supply of capsules is considerable. In addition, such devices have the further disadvantage that their performance is dependent upon the relative humidity of the atmosphere in which they are used, i.e. the quality of the cloud (the proportion of particles in the cloud which are fine enough to penetrate deep into the lung) decreases as the relative humidity increases. Furthermore, if the gelatin capsules are stored in an atmosphere of high relative humidity, they become soft and consequently difficult to handle.